1. Field of the Invention
This invention relates to medical devices in general, and more particularly to orthopedic fasteners for securing bone or soft tissue to bone.
2. Description of Related Art
Many clinically-successful treatments exist for repairing fractured bones and for re-attaching soft tissue to bone. These treatments typically involve the use of a fixation device to secure bone to bone or soft tissue to bone. By way of example, bone fractures are commonly repaired using plates and screws, cerclage wires, intramedullary rods, and external fixation devices. By way of further example, soft tissue re-attachments are commonly effected using suture anchors to suture the soft tissue to bone, headed fasteners that pierce the soft tissue and anchor in bone, and fasteners that lock the soft tissue within a bone hole.
In general, when repairing fractured bones and re-attaching soft tissue to bone, the fixation device only needs to be used in the near term. Once the bone fracture has healed or the soft tissue has biologically re-attached itself to bone, the fixation device is no longer required. In fact, the continued presence of the fixation device within the body is often detrimental to the patient, as it may interfere with the natural physiology of the patient. More particularly, in many cases, the fixation device shields the musculoskeletal structure from physiological forces. This xe2x80x9cstress shieldingxe2x80x9d typically leads to weaker tissue structures; in the case of bone tissue, for example, xe2x80x9cstress shieldingxe2x80x9d commonly leads to reduced bone mineral density, and can lead to morphological changes in cortical bone thickness and cancellous bone trabeculi.
In view of such xe2x80x9cstress shieldingxe2x80x9d concerns and other physiological concerns, and in view of the fact that many fixation devices may be palpable under the skin, and in view of the fact that many fixation devices are made from metals which can interfere with certain types of diagnostic imaging (e.g., MRI imaging), the fixation device is often removed after healing is complete. However, this necessitates a second surgery, which is accompanied by patient discomfort, risk of complications, etc.
In an effort to address the foregoing concerns, fixation devices have been fabricated from biodegradable materials. Biodegradable orthopedic fixation devices have most commonly been produced from aliphatic polyesters of poly(lactide) and poly(glycolide). However, it has been observed that in some circumstances, unfavorable histological responses can occur during the process of degradation through hydrolysis. This is particularly true when there is a large volume of material to be degraded. More particularly, a localized, sterile inflammatory response can initiate a cascade of biological events leading to osteolytic reactions which are radiographically detectable and which compromise local bone quality.
A number of devices have been developed in an attempt to mitigate the aforementioned xe2x80x9cstress shieldingxe2x80x9d issues.
U.S. Pat. No. 4,338,926 (Kummer et al.) discloses a construction in which a layer of biodegradable material is disposed between a bone plate and the bone surface. As this layer of biodegradable material degrades over time, the load carried by the bone increases and the load carried by the bone plate decreases.
U.S. Pat. No. 5,013,315 (Barrows) discloses a construction in which a layer, comprising a mix of biodegradable and non-biodegradable polymer, is disposed between a bone plate and the bone surface.
U.S. Pat. No. 5,935,127 (Border) discloses a metal intramedullary rod having apertures therein for receiving metal transfixing screws. The apertures in the metal rod are initially filled with a biodegradable polymer. During use, the transfixing screws are driven though the polymer; as the polymer resorbs, the screws become loose within the apertures, thereby shifting increasingly more load to the bone.
Japanese Patent Document No. 10085232 A (Hidekazu et al.) discloses the use of a metallic screw with a biodegrable washer. The screw and washer are used to transfix a fracture; as the washer resorbs, the compression is relieved so that the bone carries progressively more of the load.
Unfortunately, however, none of the foregoing constructions has proven entirely satisfactory, for a variety of reasons.
It would, therefore, be a significant improvement in the art to provide an orthopedic fastener which addresses the aforementioned issues of xe2x80x9cstress shieldingxe2x80x9d, bone remodeling, implant removal, etc.
Accordingly, one object of the present invention is to provide a new and improved orthopedic fastener.
Another object of the present invention is to provide a novel orthopedic fastener which is engineered to substantially eliminate bone and/or soft tissue compression after bone and/or soft tissue healing is complete.
And another object of the present invention is to provide a novel orthopedic fastener which minimizes the quantity of biodegradable polymer which must be resorbed by the body.
Still another object of the present invention is to provide a novel orthopedic fastener which is made, in part, from a material which enables bone incorporation or bone replacement.
Yet another object of the present invention is to provide a novel orthopedic fastener which facilitates future orthopedic repairs by inducing bone replacement in the bone area initially occupied by the orthopedic fastener.
And another object of the present invention is to provide a novel method for attaching objects, including bone and/or soft tissue, to bone.
These and other objects are addressed by the present invention which comprises the provision and use of a novel orthopedic fastener. The novel orthopedic fastener comprises a body portion adapted to extend substantially below the surface of a bone, and a head portion adapted to extend substantially above the surface of the bone, wherein the body portion is formed of a first material and the head portion is formed of a second material.
In one preferred form of the invention, the first material comprises a bioabsorbable material which is bioactive so as to encourage tissue in-growth, and the second material comprises a bioabsorbable material which is not bioactive so as to not encourage tissue in-growth.